Optical memory technology employing an optical disk medium with a pit pattern as a high-density and large-capacity information storage medium has been put to practical use while expanding its uses to digital audio disks, video disks, document file disks, and data file disks. Functions which are necessary for successfully carrying out information recording to an optical disk medium and information reproduction from an optical disk medium with a high reliability, by using fine-apertured laser light, are generally classified into: a converging function of forming a diffraction-limited light spot; a focal point control (focus servo) function and a tracking control (tracking servo) function of an optical system; and a pit signal (information signal) detection function.
In order to further increase the recording density of an optical disk medium, the numerical aperture NA of an objective lens which is mounted in an optical head has been increased and the wavelength λ of light from a light source has been reduced, whereby the spot diameter of light which is converged by the objective lens has been reduced. Moreover, in order to further increase the recording capacity of an optical disk medium, it has become common practice to provide the optical disk medium with a plurality of recording layers for recording information.
For example, under the CD (Compact Disc) standard, the numerical aperture NA of the objective lens is supposed to be 0.45 and the wavelength of light from the light source is supposed to be 780 nm. On the other hand, under the DVD (Digital Versatile Disc) standard, where higher recording density and larger capacity are intended, the numerical aperture NA is supposed to be 0.6 and the light wavelength is supposed to be 650 nm.
An aberration which occurs due to a tilt of an optical disk medium with respect to the optical axis increases as the light wavelength becomes shorter. Therefore, in order to obtain a good light spot while employing a shorter-wavelength light source, it is effective to reduce the base thickness (i.e., the distance from the light-entering surface of an optical disk medium to its recording layer) so as to cancel such aberration. Thus, while the base thickness is 1.2 mm in a CD, it is 0.6 mm in a DVD.
Furthermore, under the BD (Blu-ray Disc) standard employing blue laser light, the numerical aperture NA is supposed to be 0.85 and the base thickness is supposed to be 0.1 mm.
Referring to FIG. 9, a spherical aberration associated with base thickness will be described. FIG. 9 is a diagram schematically showing a spherical aberration associated with base thickness.
Laser light 911 which has been converged by an objective lens 910 is transmitted through a cover layer 903 of an optical disk medium 901, and converges at a focal point on a recording layer 902. Since the laser light 911 is refracted when traveling through the cover layer 903, as the thickness of the cover layer 903 changes, a discrepancy occurs between the focal point of the laser light 911 which has traveled near the central portion of the objective lens 910 and the focal point of the laser light 911 which has traveled near the outer periphery of the objective lens 910 (that is, a spherical aberration occurs). Such spherical aberration associated with base thickness is in proportion with the numerical aperture NA to the fourth power. Therefore, in order to prescribe a large numerical aperture NA of 0.85, as under the BD standard, a means for correcting spherical aberration is provided in the optical system.
Moreover, in order to increase the recording capacity per optical disk medium, a double-layered disk having two recording layers is also adopted under the DVD standard. Even in the case where the numerical aperture NA is to be increased, it is effective to adopt a double-layered disk structure in order to increase the recording capacity per optical disk medium.
A double-layered disk is structured so as to have, from the optical head-side, a base material, an L0 layer (first recording layer), an intermediate layer, an L1 layer (second recording layer), and a protection layer on the rear face. The base material and the intermediate layer are composed of a transparent medium such as resin. Since the intermediate layer exists between the L0 layer and the L1 layer, the thickness from the surface of the optical disk medium at the optical head side to the L1 layer is thicker than the thickness to the L0 layer, corresponding to the thickness of the intermediate layer. Since the magnitude of spherical aberration varies with the base thickness, the magnitude of spherical aberration changes when the focal point position of laser light is moved from the L0 layer to the L1 layer. However, under the DVD standard where the numerical aperture NA is 0.6, the amount of such change in spherical aberration will fall within the tolerable range, and therefore it is possible to carry out information recording/reproduction without performing aberration correction.
In the case where an objective lens having a numerical aperture NA as large as 0.8 or more is used in order to further enhance recording density, the spherical aberration associated with the thickness of the intermediate layer is no longer negligible. In other words, without spherical aberration correction, it is impossible to perform information recording/reproduction for both recording layers with a single optical head. When increasing the numerical aperture NA to be 0.8 or more, as described above, correction of spherical aberration is necessary even in the case of performing information recording/reproduction for a single recording layer. Naturally, in the case of performing information recording/reproduction for an optical disk medium having two or more recording layers, too, it is necessary to perform an optimal spherical aberration correction for each recording layer, thus to eliminate the spherical aberration associated with the thickness of the intermediate layer.
Therefore, when moving the focal point of laser light from one recording layer to another recording layer (hereinafter referred to as an “interlayer jump”) along the optical axis direction of the objective lens, the state of aberration correction must also be changed.
For example, Patent Document 1 discloses an apparatus which sets the aberration of laser light so that a spherical aberration which will occur when the focal point rests on a recording layer that is to be reached after a focus jump (hereinafter referred to as the “target recording layer”) will be corrected to some extent, this being performed before beginning an interlayer jump in a manner not allowing the current focus servo to fail. The reason why the correction can only be done to some extent is that, before beginning an interlayer jump, if the aberration of the laser light is previously set so that the spherical aberration to occur when the focal point rests on the target recording layer will be well-corrected, then the current focus servo will fail.
In order to employ a common optical head to perform information recording/reproduction for optical disk media with different laser light wavelengths and base thicknesses, e.g., CDs, DVDs, BDs, it is necessary to determine the type of the optical disk medium before a recording/reproduction operation.
Patent Document 2 discloses a medium determination method which supports CDs and DVDs. According to this method, based on whether a peak value of the focus error signal has exceeded a threshold value or not, it is determined whether an optical disk medium that is mounted in the apparatus is a DVD or a CD. When an operation of determining the type of the optical disk medium is performed in a state which is suitable for DVDS, if the mounted optical disk medium is determined to be a DVD, focus servo control is performed based on the peak values of the focus error signal as it is. If the mounted optical disk medium is a CD, the lens is again moved with a moving velocity corresponding to a CD linear velocity, and focus servo control is performed based on the peak values of the detected focus error signal.
Patent Document 3 discloses an apparatus which corrects spherical aberration by, using a micromirror array as an aberration correcting means, tilting the angle of each micromirror so as to adjust the angle of radiation of light entering the objective lens. Moreover, Patent Document 3 discloses a method for creating a plurality of focal points by adjusting the tilt angles of the micromirror array.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-16660
[Patent Document 2] Japanese Laid-Open Patent Publication No. 9-106617
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2002-288873